1. Field of the Invention
The present invention relates generally to a semiconductor integrated circuit (IC) device. More particularly, the present invention relates to a semiconductor IC device comprising dummy bonding wires which prevent shorts caused by wire sweep during the molding operation.
2. Background of the Related Art
Semiconductor integrated circuit (IC) devices require a means for electrically connecting the device to an external appliance. The electrical connection is typically accomplished by bonding wires which connect individual bonding pads on the device to respective leads of a substrate, such as a lead frame or printed circuit board. The bonding wires are usually made from gold, aluminum, or alloys thereof.
Continuing advancements in IC design and manufacturing have increased the integrity of semiconductor IC devices while decreasing the size of the devices. Thus, the numbers of leads and bonding pads are increasing while the size and pitch of the pads and the width and pitch of the leads are being reduced accordingly. However, reductions in the lead pitch are limited by device manufacturing conditions. In other words, the pitch is limited by mechanical manufacturing constraints.
In order the overcome such manufacturing constraints, the separation between the leads and the chip is increased so that more leads can be arranged around the chip, thus requiring an increase in the length of the bonding wires which connect the leads to the chip. A disadvantage of the long bonding wire is that they are more vulnerable to being displaced or dragged by the flow of molding resin entering the mold cavity during a transfer molding process. Such displacement, so-called wire sweep, causes adjacent bonding wires to contact each other and short the device.
Prevention of wire sweep during a transfer molding process using long bonding wires is an important requisite to reducing the size of the device. By reducing the device size, the number of individual devices per wafer is increased which results in increased productivity and a decrease in production costs. Although wire sweep is a common problem in the manufacture of all kinds of semiconductor IC devices, its greatest impact is on thin packages and multi-pin packages having a large number of I/O pins.
Current mass production techniques use bonding wires have a maximum length of 200 mils. The 200 mil limit results from molding operation constraints, not the wire bonding operation itself. In other words, although the wire bonding operation allows use of bonding wires that are 250 mils long, the wire sweeping caused by the molding resin flow during the molding operation prevents use of wires greater than 200 mils long.
Wire sweep in an existing semiconductor IC device will be described with reference to FIG. 1 through FIG. 4. FIG. 1 is a plan view depicting a conventional semiconductor IC device; FIG. 2 is a cross-sectional view taken along the line II--II in FIG. 1; FIG. 3 is a plan view depicting wire sweep caused by the flow of molding resin; and FIG. 4 is a cross-sectional view taken along the line IV--IV in FIG. 3, which depicts the location of bonding wires before and after the wire sweep occurs.
The IC device shown in FIG. 1 is a multi-pin package which is commonly called a QFP (Quad Flat Package). As depicted, the device has completed the wire bonding operation in which chip 10 is connected to leads of the lead frame 20 via bonding wires 30, and is ready for the subsequent molding operation. The chip 10 is attached to die pad 22 of lead frame 20, and the die pad 22 is coupled to the lead frame 20 via a plurality of, for example four (4), tie-bars 26 formed at corners of the lead frame 20. Leads 24 of the lead frame 20 are electrically connected to respective ones of corresponding bonding pads 12 of the chip 10 via bonding wires 30.
In the molding operation, the chip, electrical connections and die pad are encapsulated by a molding resin. The area inside the dashed line 40 will be encapsulated. FIG. 2 is a cross-sectional view taken along the line II--II in FIG. 1 and shows the device located in a mold 50. The gate 52, into which the molding compound enters, is located near one of four tie-bars 26 in FIG. 1. The molding resin flow enters into the mold cavity via the gate 52 in a flow direction indicated by arrows labeled 42 and fills the cavity 54 formed by upper and lower mold halves 50a, 50b.
Molding resin is a highly viscous fluid, and the bonding wires flex or are dragged in the direction of molding resin flow entering into the mold cavity 54. The resulting wire sweep caused by the molding resin flow 42 is shown in FIG. 3. FIG. 3 shows that most of bonding wires 30 bend or are displaced due to the force of the molding resin flow. In particular, the wire 30a (`outermost wire 30a`) closest to the tie-bar 26 is subjected to the most severe bending forces such that it contacts adjacent wire 30b and shorts the device (at location `S` in FIG. 3).
In FIG. 4, which is a cross-sectional view taken along the lines IV--IV in FIG. 1 and FIG. 3, the dislocation of bonding wires before and after the molding operation graphically shows the wire sweep. The location of the bonding wire before the molding operation is indicated by a solid line, while the location of the bonding wire after the molding operation is indicated by dotted line. As shown in FIG. 4, the dislocation of the outermost wire 30a is significantly greater than that of other wires 30b, 30c causing the outermost wire 30a to contact and short adjacent wire 30b. The remaining other wires 30b, 30c undergo an approximately equal amount of dislocation, which is less than the dislocation of wire 30a, causing no contact or short.
It is believed the reason why the outermost wire 30a undergoes the greatest dislocation is as follows. The distance (d.sub.1), which is the separation across a tie-bar 26 between adjacent outermost bonding wires, is greater than the distance (d.sub.2), which is the separation between adjacent bonding wires 30b and 30c. Because of the orientation of the tie-bar 26, the outermost wire 30a contacts a larger amount of the molding resin and is thus subjected to a greater bending force from the molding resin flow entering into the mold cavity than are the other wires 30b, 30c. The degree of sweep, which indicates the amount of sweep of individual bonding wires, is 4-6% for the outermost wire 30a and 2-3% for other wires 30b, 30c. Herein, the term `degree of sweep` is defined as: EQU (Displacement at center of wire/wire length).times.100.
For the IC device shown in FIGS. 1 to 4, the pitch of the bonding pads 12 is 75 .mu.m, the pitch of the leads 24 is 200 .mu.m (based on the inner end of the leads), and the distance between adjacent wires is approximately 136.5 .mu.m at their centers. In addition, the length of the outermost wire 30a is 218 mil. Accordingly, if the displacement rates of the outermost wire 30a and of its adjacent wire 30b are 6% and 3%, respectively, the displacements of the outermost wire 30a and its adjacent wire 30b are 13 mils (.apprxeq.330 .mu.m) and 6.5 mils (.apprxeq.165 .mu.m), respectively. Thus, the displacement difference (165 .mu.m) between these two wires 30a and 30b is far greater than the non-displaced distance (136.5 .mu.m) between them, resulting in wire contact.
To avoid this wire sweep problem, semiconductor IC device manufacturers generally increase the pitch of bonding pads at corners of the chip so as to provide a sufficiently large space between adjacent wires to avoid wire contact or shorts, even if wire sweep occurs. A disadvantage of this approach however, is that it runs counter to the desired trend of reducing chip size.
Another approach is taught in U.S. Pat. No. 5,302,850 to Hara, which includes a modification to the structure of the mold cavity. As shown in FIG. 5, the inflow openings 62a and 62b are centrally located on upper and lower mold portions of mold 60, respectively, whereas the existing molds such as the mold shown in FIG. 2 have an inflow opening within the confines of the lower mold portion. According to the discussion in U.S. Pat. No. 5,302,850, the direction of resin flow and the direction of the bonding wires 64 connecting the semiconductor IC device with the leads are all approximately in radial conformance from the center of the device, thereby preventing wire sweep.
However, this approach also has a disadvantage in that, after the molding operation is carried out, the resulting package body has a bur at the center of its top and bottom surfaces at the inflow opening area. These burs cause problems in a subsequent marking operation in which a trade name and/or the name of the manufacturer are printed on the surface of the package. Another disadvantage of this approach is that it requires new molding equipment.